Atom-substrate interactions

Several interactions are fundamentally important for understanding the dynamics of atomic processes on surfaces. These are  

interaction between an adsorbed atom and the substrate of different atomic structures, or site specific binding energy of an adatom  

interaction between an adatom and an impurity atom in the substrate  

These interactions are vital to understanding the dynamics of many surface phenomena, such as cluster formation and diffusion, crystal growth, surface reactivity, adsorption and desorption, and many others. 

---

Fig. 9.8 - The formation of an ad-atom by the reduction of a solvated metal ion in solution. The ad-atom may still retain a partial solvation sphere since there can be a dipole associated with the ad-atom/substrate interaction.

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Since the interaction energy /s with the finite substrate is not easy to handle due to the loss of two dimensional periodicity assumed in the derivation of Eq. 5, we simplify the substrate interaction. We considered that the substrate was a sandwich of the amorphous and the crystalline layers, and the attractive potential Uo(z) works at any point (x,y), while the translational barrier Ui(z) cos(2nx/k) only works on the crystalline substrate. The implicit assumption is that the atomic densities of the crystal and the amorphous are not so different. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

The system we wish to investigate consists of a single atom a interacting with a semi-infinite monatomic chain. The site (bond) energy of the chain is a( ), while that of the a-atom is aa (fja). Initially, the substrate is represented by a cyclic chain of N atoms, whose GF is given by (2.49). A semi-infinite chain is then formed by breaking the bond between the n = 0 and N — 1 atoms (Fig. 3.1). 

The fundamental issues to be addressed in the process modeling include spray enthalpy, gas consumption, spray mass distribution, microstructure of solidified droplets, and droplet-substrate interactions. The effects of atomization gas chemistry, alloy composition and operation conditions on the resultant droplet properties are also to be investigated in the process modeling. 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

Note how the key interaction boosting the catalytic effect is the protonation of the carbonyl group on the TG. Such catalyst-substrate interaction increases the electrophilicity of the adjacent carbonyl carbon atom, making it more susceptible to nucleophilic attack. Compare this to the base-catalyzed mechanism where the base catalyst takes on a more direct route to activate the reaction, creating first an alkoxide ion that directly acts as a strong nucleophile (Figure 4). Ultimately, it is this crucial difference, i.e., the formation of a more electrophilic species (acid catalysis) v.s. that of a stronger nucleophile (base catalysis), that is responsible for the differences in catalytic activity. 

---